Anaerobic digestion process

Abstract

Methane gas produced according to an anaerobic digestion of organic wastes is introduced into digested separated water which is alkalized by a photo-culture of algae, whereby impurity carbon dioxide gas is removed by absorption. Accordingly, methane of high concentration is recovered and at the same time, by-product algae is recycled as feedstock, and thereby improving the yield of methane.

What is claimed is: 
     
       1. A process for the treatment of organic wastes, comprising the steps: (a) anaerobically digesting organic wastes at a temperature of 30°-75° C. and a pH of 6.5-7.5 to obtain a digested slurry and fermentation gas containing methane or hydrogen, carbon dioxide and hydrogen sulfide;   (b) subjecting the digested slurry obtained in step (a) to a solid/liquid separation treatment to obtain separated digested water and digested sludge;   (c) adding calcium hydroxide to a part of the separated digested water obtained in the step (b) until the water becomes alkaline;   (d) contacting the alkaline digested water obtained in step (c) with the fermentation gas obtained in step (a) to absorb carbon dioxide and hydrogen sulfide in the digested water to produce a purified methane gas and thereby increase the concentration of hydrogen or methane in the gas;   (e) supplying a part of the digested water obtained in step (d) to a culturing system wherein photo-synthetic algae is cultured under illumination, thereby obtaining oxygen and algae culture, said culturing system being conducted at a pH of 7.5-9.5 and at a temperature of 30°-40° C.;   (f) subjecting the algae culture liquid obtained in step (e) to a solid/liquid separation treatment for obtaining algae and algae culture liquid supernatant;   (g) recycling and adding part of the algae culture liquid supernatant to the alkaline digested water of step (d) for absorbing carbon dioxide and hydrogen sulfide from the fermentation gas;   (h) subjecting additional alkaline algae culture liquid supernatant obtained in step (f) to an aerobic microbial treatment to reduce the BOD; and   (i) recycling algae obtained in step (f) into step (a) as feedstock for fermentation.   
     
     
       2. A process according to claim 1, wherein the digestion of the organic wastes under anaerobic conditions in step (a) is carried out by liquefaction fermentation with facultative anaerobic bacteria and gasification fermentation with obligatory anaerobic bacteria, thereby producing fermentation gas containing carbon dioxide and hydrogen sulfide. 
     
     
       3. A process according to claim 1 or 2, wherein sunlight is utilized as the illumination source in step (e) and the sunlight is filtered through a layer of depickled water, thereby warming the water layer, said warmed water being circulated to a heat-exchanger for heating the organic wastes feedstock, to a heat-exchanger for heating organic wastes in the digesting step and to a heat-exchanger for drying digested sludge. 
     
     
       4. A process according to claim 4, wherein said purified methane gas has a concentration exceeding 90%. 
     
     
       5. A process according to claim 4, wherein said purified methane gas has a concentration of about 99%. 
     
     
       6. A process according to claim 4, wherein said fermentation gas contains 45-70% methane and 30-55% carbon dioxide. 
     
     
       7. A process according to claim 4, wherein the concentration of carbonic acid in said algae culturing step is reduced and said algae culturing liquid is alkaline. 
     
     
       8. A process according to claim 4, wherein said purified methane gas is stored without additional desulfurization treatment. 
     
     
       9. A process according to claim 4, wherein oxygen is produced from said culturing step. 
     
     
       10. A process according to claim 9, wherein said oxygen has a purity greater than 70%. 
     
     
       11. A process according to claim 4, wherein sunlight is passed through a depickled water layer to illuminate the algae culture liquid. 
     
     
       12. A process according to claim 11, wherein heat absorbed in said depickled water layer is utilized in a heat-exchanger. 
     
     
       13. A process according to claim 4, wherein aqueous digested liquid from step (b) and alkaline aqueous digested liquid from step (d) are utilized as algae culture liquid in step (e). 
     
     
       14. A process according to claim 4, wherein calcium hydroxide is added in step (c) to adjust the pH to 7.5-10. 
     
     
       15. A process according to claim 4, wherein said fermentation gas is contacted with said alkaline digested liquid by blowing the gas into the liquid. 
     
     
       16. A process according to claim 15, wherein as a result of said blowing, the pH of the alkaline digested liquid is lowered to pH 7.2-8.0.